#include "fstream"
#include "vector"
#include "iomanip"

double solve(double x1, double y1, double x2, double y2)
{
    return x1 * y2 - x2 * y1;
}

int main()
{
	std::ifstream in("in.txt");
    std::ofstream out("out.txt");
	std :: vector<double> table_x;
	std :: vector<double> table_y;
	double x, y;
	while (in >> x >> y)
	{
		table_x.push_back(x);
		table_y.push_back(y);
	}
	double ans = 0;
	for (int i = 0; i < table_x.size(); i++)
	{
		int j = (i + 1) % table_x.size();
		ans = ans + solve(table_x[i],table_y[i],table_x[j],table_y[j]);
	}
	ans = ans / 2;
	out << std::setprecision(21) << std::abs(ans);
	return 0;
}